In recent years, the anesthesiologist's job has become increasingly complex with the addition of numerous independent monitoring systems and alarms which must be monitored in order to maintain the patient in the desired physiologic condition during an operation. It would be extremely desirable, therefore, to be able to provide the anesthesiologist with a single integrated system which can be used to monitor the patient. Such a system would contain an automatic, closed loop controller for the administration of a volatile gas anesthetic. In addition to the automatic control of the anesthetic, such a system should analyze the patients' physiologic and/or neurological condition in order to advise the anesthesiologist and to automatically maintain a preset anesthetic level or concentration.
The critical first step toward the development of the above-described system is an effective end-tidal respiratory monitor which is capable of determining and displaying the inspired/expired concentration of CO.sub.2, O.sub.2, and the volatile anesthetic gases. End-tidal gas concentrations provide relative estimates of gas concentrations in the blood and can be used as the feed-back signal for a closed anesthesia servo controller.
Numerous systems for controlling the delivery of anesthetic and monitoring of physiologic conditions have been disclosed in prior patents. For example, U.S. Pat. Nos. 3,799,149; 3,895,630; 4,233,842; 4,440,177; 4,233,842 and 4,368,740 describe various types of systems for monitoring physiologic parameters of a patient during the administration of an anesthetic. In addition, U.S. Pat. No. 3,910,261 discloses an end tidal gas analysis apparatus for analyzing concentrations of CO.sub.2 in a patient's expired gases. Despite the advances shown in the above-mentioned references, however, the prior art has heretofore been lacking an effective method and apparatus for accurately determining the end tidal concentration of expired gases. In particular, there is a need for an effective monitoring system wherein a plurality of respiratory expired gas channels can be monitored based on the accurate determination of end tidal values for a single channel of expired gas, e.g., CO.sub.2. The method and apparatus of the present invention, described in greater detail hereinbelow, fulfills this need.